maybe the Arduino UNO can be use, like you wrote the ym2149 is only a tone generator and a noise generator, so every fx and you want to do need to be software made, and they have to be performed with precision or you will be able to hear it, so things like eg. sid sound will be hard to do over USB and even harder to do over midi. you could probably make it play YM files over the USB, but i don't know if it can be done using midi.

so what i would do (if it was me), is to make a (YM player) or a (custom player) on a micro controller and just dump music files on to it,and the control start/stop from midi or usb.if you want to let it work as a midi synth you need to do your own midi interface, again you need a micro for that.and SndH is also out of the way, it is even more crazy, there you need to emulate the 68000 and 68901 MPF (or have the them as real hardware)

Atari will rule the world, long after man has disappeared

sometime my English is a little weird, Google translate is my best friend